Ultra-Fast and Scalable Saline Immersion Strategy Enabling Uniform Zn Nucleation and Deposition for High-Performance Zn-Ion Batteries.

Although aqueous Zn-ion batteries (ZIBs) with low cost and high safety show great potential in large-scale energy storage system, metallic Zn anode still suffers from unsatisfactory cycle stability due to unregulated growth of Zn dendrites, corrosion, and formation of various side products during electrochemical reaction. Here, an ultrafast and simple method to achieve a stable Zn anode is developed. By simply immersing a Zn plate into an aqueous solution of CuSO4 for only 10-60 s, a uniform and robust protective layer (Zn4 SO4 (OH)6 ·5H2 O/Cu2 O) is formed on commercial Zn plate (Zn/ZCO), which enables uniform electric field distribution and controllable dendrite growth, leading to a long-term cycle life of over 1400 h and high average Coulombic efficiency (CE) of 99.2% at 2.0 mA cm-2 and 2.0 mAh cm-2 . These excellent characteristics of the prepared Zn anode show great potential in practical applications for high-performance aqueous Zn-ion batteries.

Activating Layered Metal Oxide Nanomaterials via Structural Engineering as Biodegradable Nanoagents for Photothermal Cancer Therapy.

Layered metal oxides including MoO3 and WO3 have been widely explored for biological applications owing to their excellent biocompatibility, low toxicity, and easy preparation. However, they normally exhibit weak or negligible near-infrared (NIR) absorption and thus are inefficient for photo-induced biomedical applications. Herein, the structural engineering of layered MoO3 and WO3 nanostructures is first reported to activate their NIR-II absorption for efficient photothermal cancer therapy in the NIR-II window. White-colored micrometre-long MoO3 nanobelts are transformed into blue-colored short, thin, defective, interlayer gap-expanded MoO3-x nanobelts with a strong NIR-II absorption via the simple lithium treatment. The blue MoO3-x nanobelts exhibit a large extinction coefficient of 18.2 L g-1 cm-1 and high photothermal conversion efficiency of 46.9% at 1064 nm. After surface modification, the MoO3-x nanobelts can be used as a robust nanoagent for photoacoustic imaging-guided photothermal therapy to achieve efficient cancer cell ablation and tumor eradication under irradiation by a 1064 nm laser. Importantly, the biodegradable MoO3-x nanobelts can be rapidly degraded and excreted from body. The study highlights that the structural engineering of layered metal oxides is a powerful strategy to tune their properties and thus boost their performances in given applications.

Wrapping Plasmonic Silver Nanoparticles inside One-Dimensional Nanoscrolls of Transition-Metal Dichalcogenides for Enhanced Photoresponse.

The low light absorption of transition-metal dichalcogenide (TMDC) nanosheets hinders their application as high-performance optoelectronic devices. Rolling them up into one-dimensional (1D) nanoscrolls and decorating them with plasmonic nanoparticles (NPs) are both effective strategies for enhancing their performance. When these two approaches are combined, in this work, the light-matter interaction in TMDC nanosheets is greatly improved by encapsulating silver nanoparticles (Ag NPs) in TMDC nanoscrolls. After the silver nitrate (AgNO3) solution was spin-coated on monolayer (1L) MoS2 and WS2 nanosheets grown by chemical vapor deposition, Ag NPs were homogeneously formed to obtain MoS2-Ag and WS2-Ag nanosheets due to the TMDC-assisted spontaneous reduction, and their size and density can be well controlled by tuning the concentration of the AgNO3 solution. By the simple placement of alkaline droplets on MoS2-Ag or WS2-Ag hybrid nanosheets, MoS2-Ag or WS2-Ag nanoscrolls with large sizes were obtained in large area. The obtained hybrid nanoscrolls exhibited up to 500 times increased photosensitivities compared with 1L MoS2 nanosheets, arising from the localized surface plasmon resonance effect of Ag NPs and the scrolled-nanosheet structure. Our work provides a reliable method for the facile and large-area preparation of NP/nanosheet hybrid nanoscrolls and demonstrates their great potential for high-performance optoelectronic devices.

Boosting Electrocatalytic Activity of 3d-Block Metal (Hydro)oxides by Ligand-Induced Conversion.

The 3d-transition-metal (hydro)oxides belong to a group of highly efficient, scalable and inexpensive electrocatalysts for widespread energy-related applications that feature easily tailorable crystal and electronic structures. We propose a general strategy to further boost their electrocatalytic activities by introducing organic ligands into the framework, considering that most 3d-metal (hydro)oxides usually exhibit quite strong binding with reaction intermediates and thus compromised activity due to the scaling relations. Involving weakly bonded ligands downshifts the d-band center, which narrows the band gap, and optimizes the adsorption of these intermediates. For example, the activity of the oxygen evolution reaction (OER) can be greatly promoted by ≈5.7 times over a NiCo layered double hydroxide (LDH) after a terephthalic acid (TPA)-induced conversion process, arising from the reduced energy barrier of the deprotonation of OH* to O*. Impressively, the proposed ligand-induced conversion strategy is applicable to a series of 3d-block metal (hydro)oxides, including NiFe2 O4 , NiCo2 O4 , and NiZn LDH, providing a general structural upgrading scheme for existing high-performance electrocatalytic systems.

Thick Two-Dimensional Water Film Confined between the Atomically Thin Mica Nanosheet and Hydrophilic Substrate.

The interesting properties of water molecules confined in a two-dimensional (2D) environment have aroused great attention. However, the study of 2D-confined water at the hydrophilic-hydrophilic interface is largely unexplored due to the lack of appropriate system. In this work, the behavior of water molecules confined between an atomically thin mica nanosheet and a hydrophilic SiO2/Si substrate was investigated using an atomic force microscope in detail at ambient conditions. The confined water molecules aggregated as droplets when the relative humidity (RH) of the environment was 11%. A large-area 2D water film with a uniform thickness of ∼2 nm was observed when the mica flake was incubated at 33% RH for 1 h before being mechanically exfoliated on a SiO2/Si substrate. Interestingly, the water film showed ordered edges with a predominant angle of 120°, which was the same with the lattice orientation of the mica nanosheet on top of it. The water film showed a fluidic behavior at the early stage and reached a stable state after 48 h under ambient conditions. The surface properties of the upper mica nanosheet and the underlying substrate played a crucial role in manipulating the behavior of confined water molecules. When the surface of the upper mica nanosheet was modified by Na+, Ni2+, and aminopropyltriethoxysilane (APS), only some small water droplets were observed instead of a water film. The surface of the underlying SiO2/Si substrate was functionalized by hydrophilic APS and hydrophobic octadecyltrimethoxysiliane (OTS). The small water droplets were imaged on a hydrophobic OTS-SiO2/Si substrate, while the water film with regular edges was maintained on a hydrophilic APS-SiO2/Si substrate. Our results might provide an alternative molecular view for investigating structures and properties of confined water molecules in 2D environments.

Highly Doped Carbon Nanobelts with Ultrahigh Nitrogen Content as High-Performance Supercapacitor Materials.

Nitrogen-doped and nitrogen and oxygen codoped carbon nanobelts (CNBs) (denoted as N-CNBs and N-O-CNBs, respectively) are respectively obtained by pyrolyzing the self-aligned polypyrrole (PPy) NBs and Se@poly(2-methoxy-5-nitroaniline) core@shell nanowires. Particularly, the uniform size, unique nanostructure, and well-defined edges of the PPy NBs result in the uniform size of the doped CNBs with an extraordinarily high N doping level (≈16 at%), especially the very large concentrations of the redox active pyridinic (9 at%) and pyrrolic N (3.5 at%) species. Furthermore, the precursors in highly self-aligned, dense arrays give rise to a very high packing density for the N-CNBs and N-O-CNBs. These incomparable features provide not only appropriate pathways for the introduction of pseudocapacitance via rapid Faradaic reactions and enhancement of volumetric capacitance but also structural design and synthesis approach to new types of nanostructured carbon. Notably, the N-CNBs obtained at the pyrolysis temperature of 800 °C (N-CNB8) in symmetric electrochemical cells deliver a specific capacitance of 458 F g-1 and ultrahigh volumetric capacitance of 645 F cm-3 in aqueous solution, which are among the best performance ever reported for carbon-based supercapacitive materials.

Probing Polymorphic Stacking Domains in Mechanically Exfoliated Two-Dimensional Nanosheets Using Atomic Force Microscopy and Ultralow-Frequency Raman Spectroscopy.

As one of the key features of two-dimensional (2D) layered materials, stacking order has been found to play an important role in modulating the interlayer interactions of 2D materials, potentially affecting their electronic and other properties as a consequence. In this work, ultralow-frequency (ULF) Raman spectroscopy, electrostatic force microscopy (EFM), and high-resolution atomic force microscopy (HR-AFM) were used to systematically study the effect of stacking order on the interlayer interactions as well as electrostatic screening of few-layer polymorphic molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) nanosheets. The stacking order difference was first confirmed by measuring the ULF Raman spectrum of the nanosheets with polymorphic stacking domains. The atomic lattice arrangement revealed using HR-AFM also clearly showed a stacking order difference. In addition, EFM phase imaging clearly presented the distribution of the stacking domains in the mechanically exfoliated nanosheets, which could have arisen from electrostatic screening. The results indicate that EFM in combination with ULF Raman spectroscopy could be a simple, fast, and high-resolution method for probing the distribution of polymorphic stacking domains in 2D transition metal dichalcogenide materials. Our work might be promising for correlating the interlayer interactions of TMDC nanosheets with stacking order, a topic of great interest with regard to modulating their optoelectronic properties.

Highly Sensitive Temperature Detection Based on a Frost- and Dehydration-Resistive Ion-Doped Hydrogel-MXene Composite.

Wearable temperature sensors with high sensitivity and stability hold great potential for human health monitoring. However, hydrogels, which are commonly used for wearable devices, often show poor thermal and electrical conductivity and are susceptible to dehydration and freezing. Herein, we developed a frost- and dehydration-resistive temperature sensor based on Fe2+/Ti2CTx/κ-carrageenan (CA)-polyacrylamide (PAM) hydrogel. The Fe2+ ions within the hydrogel existed in two forms: as free ions and bonded ions. The free Fe2+ ions could complex with water molecules, resulting in the improved resistance to dehydration and freezing, as well as enhanced ionic conductivity in the hydrogel. On the other hand, the remaining Fe2+ ions acted as linkers to form coordination bonds with the sulfate groups of CA chains, resulting in the greatly enhanced mechanical strength of the hydrogel. In addition, the Ti2CTx nanosheet-based fillers formed a well-defined porous laminar structure, which reduced the phonon scattering and improved the phonon adsorption within the hydrogel. The Fe2+/Ti2CTx/CA-PAM hydrogel sensor exhibited excellent temperature sensing performance including a good linearity (R2 = 0.998) within a broad working range (-10 to 60 °C), high resolution (0.1 °C), and good repeatability. Furthermore, the sensor was integrated into a wireless system for continuous monitoring of body temperature, demonstrating its potential in healthcare monitoring, electronic skins, and intelligent robots.

Transition Metal Dichalcogenides Nanoscrolls: Preparation and Applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) nanosheets have shown extensive applications due to their excellent physical and chemical properties. However, the low light absorption efficiency limits their application in optoelectronics. By rolling up 2D TMDCs nanosheets, the one-dimensional (1D) TMDCs nanoscrolls are formed with spiral tubular structure, tunable interlayer spacing, and opening ends. Due to the increased thickness of the scroll structure, the light absorption is enhanced. Meanwhile, the rapid electron transportation is confined along the 1D structure. Therefore, the TMDCs nanoscrolls show improved optoelectronic performance compared to 2D nanosheets. In addition, the high specific surface area and active edge site from the bending strain of the basal plane make them promising materials for catalytic reaction. Thus, the TMDCs nanoscrolls have attracted intensive attention in recent years. In this review, the structure of TMDCs nanoscrolls is first demonstrated and followed by various preparation methods of the TMDCs nanoscrolls. Afterwards, the applications of TMDCs nanoscrolls in the fields of photodetection, hydrogen evolution reaction, and gas sensing are discussed.

Solvent-Free Preparation of Closely Packed MoS2 Nanoscrolls for Improved Photosensitivity.

Due to their enhanced light absorption efficiency, one-dimensional (1D) transition metal dichalcogenide (TMDC) nanoscrolls derived from two-dimensional (2D) TMDC nanosheets have shown excellent optoelectronic properties. Currently, organic solvent and alkaline droplet-assisted scrolling methods are popular for preparing TMDC nanoscrolls. Unfortunately, the adsorption of organic solvent or alkaline impurities on TMDC is inevitable during the preparation, which affects the optoelectronic properties of TMDC. In this work, we report a solvent-free method to prepare closely packed MoS2 nanoscrolls by dragging a deionized water droplet onto the chemical vapor deposition grown monolayer MoS2 nanosheets at 100 °C (referred to as MoS2 NS-W). The as-prepared MoS2 NS-W was well characterized by optical microscopy, atomic force microscopy, and ultralow frequency (ULF) Raman spectroscopy. After high temperature annealing, the height of MoS2 nanoscrolls prepared using an ethanol droplet (referred to as MoS2 NS-E) greatly decreased, indicating the loss of encapsulated ethanol in MoS2 NS-E. While the height of MoS2 NS-W was almost unchanged under the same conditions, implying that no water was embedded in the scroll. Compared to the MoS2 NS-E, the MoS2 NS-W shows more ULF breathing mode peaks, confirming the stronger interlayer interaction. In addition, the MoS2 NS-W shows a higher Young's modulus than MoS2 NS-E, which could arise from the closely packed scroll structure. Importantly, the MoS2 NS-W device showed a photosensitivity 1 order of magnitude higher than that of the MoS2 NS-E device under blue, green, and red lasers, respectively. The decreased photosensitivity of MoS2 NS-E was attributed to the larger dark current, which might be assigned to the adsorbed ethanol between the adjacent layers in MoS2 NS-E. Our work provides a solvent-free method to prepare closely packed MoS2 nanoscrolls at large scale and demonstrates their great potential for high-performance optoelectronic devices.

Ligand-assisted deposition of ultra-small Au nanodots on Fe2O3/reduced graphene oxide for flexible gas sensors.

The development of flexible room-temperature gas sensors is important in environmental monitoring and protection. In this contribution, by using 1-octadecanethiol (ODT) as a surface ligand, Au nanodots (NDs) with ultra-small size of ∼1.7 nm were deposited on the surface of α-Fe2O3/reduced graphene oxide (rGO). The Au ND-ODT/α-Fe2O3/rGO composite was fabricated into flexible gas sensors, which could detect NO2 gas down to 200 ppb at room temperature. Compared with α-Fe2O3/rGO, Au ND-ODT/α-Fe2O3/rGO showed enhanced sensing performance because of the beneficial effects of Au NDs, including facilitating the adsorption of NO2 molecules and forming ohmic-like contact with rGO and α-Fe2O3. In addition, the sensing performance of the composite was also influenced by the surface ligands of the Au NDs. Ligands with less polar terminal groups were found to be beneficial to charge transfer in the sensing film. Moreover, Au ND-ODT/α-Fe2O3/rGO-based flexible sensors showed negligible performance deterioration under moderately bent conditions, suggesting their potential to be used in portable and wearable devices.

Realization of Oriented and Nanoporous Bismuth Chalcogenide Layers via Topochemical Heteroepitaxy for Flexible Gas Sensors.

Most van der Waals two-dimensional (2D) materials without surface dangling bonds show limited surface activities except for their edge sites. Ultrathin Bi2Se3, a topological insulator that behaves metal-like under ambient conditions, has been overlooked on its surface activities. Herein, through a topochemical conversion process, ultrathin nanoporous Bi2Se3 layers were epitaxially deposited on BiOCl nanosheets with strong electronic coupling, leading to hybrid electronic states with further bandgap narrowing. Such oriented nanoporous Bi2Se3 layers possessed largely exposed active edge sites, along with improved surface roughness and film forming ability even on inkjet-printed flexible electrodes. Superior room-temperature NO2 sensing performance was achieved compared to other 2D materials under bent conditions. Our work demonstrates that creating nanoscale features in 2D materials through topochemical heteroepitaxy is promising to achieve both favorable electronic properties and surface activity toward practical applications.

Anion-dependent topochemical conversion of CoAl-LDH microplates to hierarchical superstructures of CoOOH nanoplates with controllable orientation.

Hierarchical superstructures of laterally or vertically oriented CoOOH nanoplates were prepared by topochemical conversion of CoAl-LDH microplates intercalated with CO32- or SO42- anions, respectively. The superstructure of vertically oriented nanoplates exhibited better electrocatalytic performance as compared to the lateral counterpart, attributable to the enlarged accessible surface area and promoted reaction kinetics.

Origin of High Efficiency and Long-Term Stability in Ionic Liquid Perovskite Photovoltaic.

Environment-friendly protic amine carboxylic acid ionic liquids (ILs) as solvents is a significant breakthrough with respect to traditional highly coordinating and toxic solvents in achieving efficient and stable perovskite solar cells (PSCs) with a simple one-step air processing and without an antisolvent treatment approach. However, it remains mysterious for the improved efficiency and stability of PSCs without any passivation strategy. Here, we unambiguously demonstrate that the three functions of solvents, additive, and passivation are present for protic amine carboxylic acid ILs. We found that the ILs have the capability to dissolve a series of perovskite precursors, induce oriented crystallization, and chemically passivate the grain boundaries. This is attributed to the unique molecular structure of ILs with carbonyl and amine groups, allowing for strong interaction with perovskite precursors by forming C=O…Pb chelate bonds and N-H…I hydrogen bonds in both solution and film. This finding is generic in nature with extension to a wide range of IL-based perovskite optoelectronics.

Impact of pH on Regulating Ion Encapsulation of Graphene Oxide Nanoscroll for Pressure Sensing.

Recently, graphene oxide nanoscroll (GONS) has attracted much attention due to its excellent properties. Encapsulation of nanomaterials in GONS can greatly enhance its performance while ion encapsulation is still unexplored. Herein, various ions including hydronium ion (H3O⁺), Fe3+, Au3+, and Zn2+ were encapsulated in GONSs by molecular combing acidic graphene oxide (GO) solution. No GONS was obtained when the pH of the GO solution was greater than 9. A few GONSs without encapsulated ion were obtained at the pH of 5-8. When the pH decreased from 5 to 0.15, high-density GONSs with encapsulated ions were formed and the average height of GONS was increased from ~50 to ~190 nm. These results could be attributed to the varied repulsion between carboxylic acid groups located at the edges of GO nanosheets. Encapsulated metal ions were converted to nanoparticles in GONS after high-temperature annealing. The resistance-type device based on reduced GONS (rGONS) mesh with encapsulated H3O⁺ showed good response for applied pressure from 600 to 8700 Pa, which manifested much better performance compared with that of a device based on rGONS mesh without H3O⁺.